In the IEEE Transactions on Communication Technology, December, 1965, pages 475-483, Ward discloses a telemetry synchronizing system wherein bit synchronization of a received signal in the form of a pseudorandom non-return to zero (NRZ) sequence, is attained followed by phase adjustment of the received signal and a locally generated signal. The prior art system includes a delay lock loop having a feedback shift register for deriving a locally generated pseudorandom NRZ signal having the same sequence as the received signal. The shift register derives a pair of time delayed output bits that are separately compared with a replica of the received signal. Typically, the received signal leads one of the output bits by one-half a bit length and lags the other bit by one-half a bit length.
Signals resulting from the comparisons are added together to derive a phase indicating analog error signal that is applied to an analog filter. The filter derives a signal to control the phase of a clock wave derived from a voltage controlled oscillator. The clock wave controls shifting of signals initially loaded into the shift register stages as an estimate of the initial bits in the locally generated pseudorandom sequence. The estimate of the initial bits is derived by loading the shift register with received bits of the input signal.
The NRZ output signal of the shift register is compared with the received NRZ signal to derive a second analog error signal which is supplied to a low pass analog filter which drives a threshold detector. In response to the threshold of the detector being reached or exceeded, the received bits are no longer loaded into the shift register and the delay lock loop corrects any initial misalignment due to the relative phases of the received signal and the voltage controlled oscillator.
It is necessary to employ the threshold detector to determine the correlation between the locally generated and received pseudorandom sequences because the delay lock loop has an ambiguity in its response. In particular, the phase indicating error signal supplied to the voltage controlled oscillator has a zero value for three different conditions; to wit: (1) when the input signal is one-half a bit displaced from the two output bits of the shift register stages; (2) when the input signal is lagging one of the output bits by one bit; and (3) when the input signal is leading the other output bit by one bit. Between these three zero points are peak positive and negative error signals that are supplied to the voltage controlled oscillator; one peak occurs when one output bit and a bit of the input signal are in phase; the second peak occurs when a bit of the received signal and the second output bit overlap. By detecting when the correlation function between the received and locally generated pseudorandom sequences exceeds a threshold value, the ambiguity is resolved.
While the prior system reported by Ward in the December, 1965 IEEE Transactions is a considerable improvement over earlier art systems for acquiring pseudorandom sequences, it appears to have certain disadvantages. In particular, the Ward system is apparently adapted for use only in connection with NRZ signals and is not adapted to be utilized with L format biphase signals. Further, it appears that the prior art system functions by initially determining if synchronization exists between the locally generated and received pseudorandom sequences and then correcting any misalignment due to the relative phases of the two received signals and the output signal of the voltage controlled oscillator. Such sequential operation is disadvantageous because it does not simultaneously achieve phase lock and synchronization, which implies less than optimum phase lock and synchronization acquisition time. It would also appear that if there is a severe misalignment between the relative phases of the received signal and the voltage controlled oscillator the correlation threshold may not be reached even when the received and locally generated pseudorandom sequences are the same.